This invention relates to the hydrogenation of phenol. More particularly, it relates to the hydrogenation of phenol to cyclohexanone in the presence of a promoted palladium catalyst.
In the hydrogenation of phenol employing a palladium catalyst, the activity of the catalyst, and hence the rate of hydrogenation, decreases with continued use of the catalyst due to impurities present in the hydrogenation reaction mixture which poison the catalyst. While processes, such as those disclosed in U.S. Pat. Nos. 3,692,845 and 3,187,050, have been developed to purify organic compounds such as phenol to be hydrogenated, the poisoning of metallic catalysts has not been entirely eliminated in large scale commercial processes due to long term accumulation of impurities, particularly impurities which are produced during the processing.
To avoid the economically prohibitive alternatives of discarding poisoned catalyst or continuing to use the poisoned catalyst at a reduced rate of hydrogenation, it is known to promote the rate of hydrogenation, thereby at least partially overcoming the disadvantages of continued use of such poisoned palladium catalysts. The hydrogenation of phenol to cyclohexanone is normally promoted by the use of "promoted palladium-on-carbon catalysts", i.e., catalysts which have been treated prior to their addition to the hydrogenation reaction mixture, to incorporate on the catalysts a material which enhances the activity of the catalyst. Thus, in U.S. Pat. No. 3,076,810, cyclohexanone is produced by hydrogenating phenol using a sodium-promoted palladium catalyst which has been modified prior to its introduction to the reaction mixture to incorporate sodium thereon. Alkaline reacting agents in limited amounts are also disclosed as being added to assist in promotion when the sodium-promoted catalysts are employed. However, such catalyst systems have not been entirely satisfactory because of long-term accumulation of catalyst poisons on the catalyst, and research has been continued to develop an improved process and/or catalyst.
Surprisingly, the process of present invention provides significantly improved catalyst selectivity and activity in the hydrogenation of phenol to cyclohexanone and mitigates long-term accumulation of catalyst poisons on the catalyst. Moreover, the improved catalyst and process minimizes the loss of palladium in commercial processes involving continuous hydrogenation of phenol. It is also important that the present process can be operated at a lower temperature than present commercial plants without sacrificing production rate.